1. Field of the Invention
This invention relates to inverters, and more particularly, to the selective actuation of a commutation circuit in a self-commutated inverter by inhibiting the operation of the commutation circuit when the main thyristors will transition to the blocked state because of the voltage-current phase relationship of the load, and the switching or modulation pattern of the inverter.
2. Description of the Prior Art
Inverters are known generally and are devices which transform DC (direct current) electrical energy, such as from a fuel cell or the like, into AC (alternating current) electrical energy suitable for use by utility companies or other consumers of electrical energy. Most inverters include at least one pair of main switching elements, and by alternatively actuating each switching element, electrical energy from the DC source flows through the load first in one direction and then in the reverse direction forming a fundamental AC waveform.
Numerous different types of switching devices can be employed in an inverter as a switching element to connect the positive and negative input buses to the load. Semiconductor switches, such as thyristors, are frequently used in present day inverters and this type of device is substantially unidirectional so that the high energy current pulses pass therethrough in only one direction when the switch is turned on by a gate signal. Some semiconductor switches, particularly those capable of handling large currents such as thyristors, will not immediately change from a conducting state to a nonconducting state upon the removal of the gate signal, but require that the instantaneous current passing therethrough be reduced to near zero, sometimes in conjunction with a reverse bias, to allow the thyristors to transition to its off state. The process by which the current is reduced to zero is known as "commutation" and numerous circuit configurations which store energy and then release it have been proposed for this function. Many commutation circuits operate by displacing the current in the solid state switch from a storage device, such as a capacitor or resonant circuit, for a time period greater than the turn off time of the semiconductor switch. During this period the load current is being supplied by the storage device, and the magnitude of the current through the thyristor drops to zero for a sufficient time period to allow the transition of the thyristor to the nonconducting state.
In inverters generally, it has been recognized that it is desirable to make the transformation between DC electrical energy and AC electrical energy as efficiently as possible for the well recognized reasons of energy consumption, cost saving, etc. One method of improving inverter efficiency is to accurately match the size of the commutation current pulse to the instantaneous value of the current to be extinguished. It is known that the required size of a commutation pulse varies directly with the magnitude of the load current, and that one efficiency improving technique is to decrease the energy stored in the commutation circuit per cycle as a function of load.
One efficiency improving variation is disclosed in U.S. patent application Ser. No. 973,339 by J. R. Vivirito filed on Dec. 26, 1978 for AUXILIARY COMMUTATION CIRCUIT FOR AN INVERTER, assigned to the same assignee as the present invention. The disclosed apparatus includes an auxiliary commutation circuit of the impulse commutated bridge inverter type in which additional commutation energy is stored on a pair of oppositely charged capacitors. Rather than operating on every cycle, switches in series with the charged capacitors are operable only in response to a sensed overcurrent condition to provide the additional stored energy for commutation.
Still another energy saving variation of interest is disclosed in U.S. patent application Ser. No. 972,543 to G. J. Messer filed on Dec. 22, 1978 for CONTROL FOR AN AUXILIARY COMMUTATION CIRCUIT, also assigned to the same assignee as the present invention. A control circuit used with an auxiliary commutation circuit of an inverter which responds to the increased time period resulting from the additional capacitance and extends the initiation of the make up pulse. This allows a natural decay of the commutation pulse so that the make up pulse begins when the instantaneous value of the current through the thyristor is low.
Yet another efficiency improving technique is disclosed in U.S. Pat. No. 3,391,328 issued July 2, 1968 to B. Mokrytzki for INCREASED EFFICIENCY COMMUTATION CIRCUIT FOR THYRISTORS. In that patent a commutation current is limited in magnitude to that which approaches the level required for safe commutation thereby minimizing wasted current and increasing efficiency.
All of the heretofore identified disclosures utilize a commutation circuit which is rendered operative on a per cycle basis to essentially extinguish the load current through the thyristor being commutated, whether or not a commutation current is actually required. Accordingly, each time the commutation circuit is triggered, some of the energy from the source is consumed by storing and discharging a part of this energy within the inverter, a process which decreases inverter efficiency.